Abstract

Have you ever had fun playing with sand and water, observing how little rivers you create carve their way down to the lowest point of the sandbox, backyard or beach? Some meander, others braid, and some carve a path straight down.
Hyrdologists (or scientists who study water) do very much the same thing! Only they set up the model in a particular way, so observing their mini-rivers helps them answer questions about how water flow affects the environment. In this geology science project, you will create your own question, set up a model with sand, cornmeal, and water to help you study your question, and learn from your observations just like a real hydrologist.

Objective

Model rivers using cornmeal, sand, and water to observe how varying water speed changes the riverbed.

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Introduction

Have you ever played in the terrain of your backyard, pouring water over dirt and rocks to create little rivers that eventually flow into a gully or gutter? You might know that water is a major element shaping the landscape around us. Think of the devastation after a flood or the destructive power of a tsunami. Water also shapes and reshapes the landscape in a much slower way, gradually over long periods of time. Take the Grand Canyon in Arizona. For millions of years, the Colorado River has carried crushed rock across a plateau, carving into it a steep valley more than a mile deep! Figure 1 shows two other examples where water has shaped the landscape.

Figure 1. Two pictures illustrate how water shapes and reshapes the land over time. Left: the Grand Canyon of the Yellowstone in Wyoming, Right: Jim River Canyon near Kanuti National Wildlife Refuge in Alaska.

Water comes from rainfall and snowmelt. Some water soaks into the ground. The rest flows across the land as runoff into rivers, streams, and lakes. On its way, water erodes the land and drags sediment and debris with it.

Not all sediments are equal. Scientists classify sediment based on its grain size, all the way from giant boulders to pebbles, gravel, sand, silt, clay, and colloid. In a colloid, the grain size is microscopically small; if you are interested in learning more about colloids check out the science project Making Mixtures: How Do Colloids Size Up. The composition of the sediment is also important.

Why is this classification of sediments important? One reason is that water with more speed, or fast-running water, has more energy and can carry bigger, heavier sediments with it.

Some of the sediment sinks and settles out on the ground. This can leave self-organized structures behind like ripples or dunes on the beach or on riverbeds. Figure 3 shows an example of sand ripples on a beach.

Figure 3.
Ripples on a beach are an example of self-organizing structures left behind when sediment settles out of flowing water.

When water from rainfall and snowmelt accumulates too quickly for the ground to absorb or the rivers to carry away, flooding happens. This type of flooding is called alluvial flooding, or river-based flooding. The flooded water retreats slowly or is absorbed by the ground, leaving sediments behind. Figure 4 shows sediment-rich runoff after a flood and the sediment left behind.

Although water tends to accumulate at the lowest point, it does not always take a straight path there. Some water will meander, as shown at left in Figure 5. The picture on the right is an example of what scientists call a braided river. Braided rivers have several small channels separated by often temporary islands.

Figure 5. This illustration shows a meander in a river (left) and a braided river (right). (Left photo courtesy of Google earth @2013 Google; right photo courtesy of Map data @2013 Google.)

Hydrologists use models in addition to outdoor observation to gather information. Models are simplified representations of an object in the real world. Some models exist on the computer, others are physical replicas. Both are handy tools to study rivers.

In this geology science project, you will build a model that represents a river in the real world. Your model will act the same as a river in nature in important ways. You will change the speed at which water runs in your river and observe how this affects other characteristics like the width of the riverbed, if and where deposits are formed or whether or not flooding occurs.

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Materials and Equipment

Empty milk/water jugs, 1 gallon size (4)

Utility knife or box cutter

Permanent marker

Adult helper

Area with small slope that is OK to get messy (at least 16 by 20 inches). Some examples are:

Sloped area in a larger sandbox, suggested dimension 20 inches long, 16 inches wide, and a slope of about 4 inches high.

A brick or big block under one side of a movable sandbox to obtain a 4-inch slope.

A driveway with a slight slope equivalent to 5 inches over a length of about 2 feet.

Plastic sheet or tarp (20 by 22 inches)

Sand (25 pounds)

Dowel, diameter approximately 1 inch, length between 10 and 20 inches. Any sturdy material is fine: PVC, wood, or the inner cardboard roll from a roll of paper towels work well.

Ruler

Pencil (3) or drinking straws (3)

Cornmeal (16 pounds)

Video and/or photo camera (optional)

Water from household or garden tap

Volunteer or helper (can be the same as the adult helper)

Notebook

Pen or pencil

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Experimental Procedure

Prepare Your Work Area and Tools

This geology science project uses a model to study rivers. The water in the model river moves much slower than water in a real river. Because of this, the water in the model cannot move real sand in the same way as it moves real sand in nature. We need to choose a lighter material to represent the sand. Hydrologists use plastic sand in their models. We will use cornmeal in this geology science project to model the sand in real rivers.

Prepare your jugs.

Ask an adult to remove the tops of the four jugs as shown in Figure 6. This enables you to pour water easily in and out of the jug.

Figure 6.
A gallon jug with the top removed using a utility knife.

Ask an adult to cut a small triangular hole near the bottom of the first jug. The sides of this triangle should be between ¼ and ½ inch long and the horizontal line of the triangle should more or less align with where the bottom of the jug touches the ground when standing. Figure 7 shows an example.

Figure 7.
Gallon jug with a triangular hole cut near the bottom.

Ask an adult to cut a medium triangular hole near the bottom of the second jug. This time, the sides of the triangle should be about ¾ inch long.

Ask an adult to cut a large triangular hole near the bottom of the third jug. This time, the sides of the triangle should be about 1-¼ inches long.

The fourth jug will be used as a watering can. Using a permanent marker, draw a line where this jug is approximately ¾ full. (Note: You will want to be more exact here if you plan on making quantitative measurements of water flow speed. Fill the jug with ¾ of a gallon of water (12 cups) and mark this water level with permanent marker on the jug.) This way - you will know you used exactly ¾ gallon for each river. Measuring the time it takes for the ¾ gallon of water to flow will allow you to calculate the water speed in gallons per seconds.

Prepare your work area.

Cover your work area with the big plastic sheet or tarp. This will make cleanup easy.

Flatten the plastic as much as possible.

Spread sand evenly over the plastic, covering an area about 16 inches wide by 20 inches long with a layer of sand roughly 1 inch thick. The 20 inch length should be on a downhill slope. In total there should be a change of 4 inches in height over the length of he area.

Use your dowel to spread and flatten the sand.

Push the dowel in the sand to make three riverbeds as shown in Figure 8.

Figure 8. Use the dowel to create riverbeds in the sand.

Create three riverbeds parallel to each other, leaving about 4 inches of space between them. Start about 2 inches from the side of your workspace.

Push the dowel in the sand so approximately half of the dowel is in the sand. This creates riverbeds of about ½ inch deep at the deepest point.

Put a pencil (or drinking straw) as shown in Figure 8 at the end of each riverbed so you remember where the riverbeds are once you cover the area with cornmeal.

Create a top layer of cornmeal.

Cut a small hole in the cornmeal bag.

Spread the cornmeal over the sand by shaking the bag. Try to pour an equal amount of cornmeal over the whole work area. This layer should be between ½ and 1 inch thick. Make sure you leave at least 4 pounds (or a fourth) of the cornmeal remaining for future testing.

Use your dowel to spread and flatten the layer of cornmeal.

Push your dowel at the same locations where you had the sand riverbeds to re-create those riverbeds. The pencils (or drinking straw) sticking up at one end will indicate where the riverbeds are.

Let the Rivers Run

Time to let the water flow! You will pour an equal amount of water on each model riverbed, but the water will run at different speeds, depending on the size of the hole cut in the bottom of the jug. You will start with the slow-running river (the jug with the smallest hole) and move on to the medium- and fast-running rivers as you move from one riverbed model to the next.

If you would like to be more quantitative about how fast each river is flowing then try timing how long it takes for each jug to empty. You can report the flow rate as the number of gallons per second.

In your lab notebook, create a table like Table 1. You will use it to record your observations.

Slow-Flowing River

Medium-Flowing River

Fast-Flowing River

Observations While Water Flows

Evidence Seen in Riverbed after Water Flows

Table 1. In your lab notebook, create a table like this one to record your observations for each set of experiments you perform.

If you plan on taking a video of your project, get everything ready for videotaping.

Position the jug with the smallest hole and the jug you will use as a watering can near the first model riverbed. To model the slow flowing river.

Fill the watering can up to the ¾ line.

Let the river run:

Hold the jug with the hole hovering over the end of the first model riverbed. The hole should be aligned with the riverbed as shown in Figure 9.

If you have the space and extra sand to create a stable platform to put your jug down, you can do so. But if you do, make sure you create platforms of the same height for the three different river models.

If you need to hold your jug a little higher because the edge of your sandbox is in the way, do so. But be sure to keep the jugs at the same height as you let the water run in the three different river models.

Let a helper pour the water from the watering can in the jug with the hole as shown in Figure 9, being sure that he or she pours the water in without stopping and interrupting the water flow into the model riverbed.

If you choose to videotape your project, begin filming now.

Figure 9.
Hold the jug so that the water flows into the model riverbed. Note that the triangular hole in the jug is well aligned with the riverbed.

Note any observations about what happened while the river was flowing in Table 1. Here some things to look for:

Did you observe any braiding, meandering, or flooding while the water flows?

Were islands created by the water in your river model? Did they stay the entire time, or get washed away?

Did the banks of your river hold firm or break down? If they broke down, did it happen gradually or suddenly?

Do you see evidence of what happened in the riverbed after water flows?

Repeat steps 4 through 6 for the jug with the medium hole (medium-flowing river) and the second riverbed model.

Repeat steps 4 through 6 for the jug with the large hole (fast-flowing river) and the third riverbed model.

Measure how wide the riverbeds are at 5-inch intervals. Create a table like Table 2 in your lab notebook. You will use it to record your measurements.

Distance from Source

Slow-Flowing River

Medium-Flowing River

Fast-Flowing River

Width(inches)

Observations

Width(inches)

Observations

Width(inches)

Observations

0 inch

5 inches

10 inches

15 inches

20 inches

Table 2. In your lab notebook, create a table like this one to record your measurements for each set of experiments you perform.

Measure the width of the riverbeds at 5-inch intervals, starting from the source (or the place where you held the jug with the hole).

Note all your measurements in your table.

Add any further observations about erosion and sedimentation you observe. Some things to look for are:

Visible sand on the riverbed shows the cornmeal bed has completely eroded away.

Pockets of cornmeal in a riverbed that looks completely eroded are a sign of sediment deposit. This is possibly a leftover island of a braided river.

Widened riverbanks are evidence of erosion.

Wet areas outside the riverbanks can be signs of flooding.

Repeat the experiment on the riverbed models two more times.

If any plunge pools were created, fill them with sand. You do not have to fill the sand layer in any other way. (Plunge pools form where vertically falling water touches the ground or the riverbed and the force creates a depression, or pool, of deep water. Plunge pools would typically form if you held your jugs higher than the riverbed.)

Add cornmeal to create a new riverbed for each river model.

Repeat steps 1 through 10.

Clean up your work area.

Dispose of all the cornmeal and sand on the plastic sheet or tarp. This material can go to the compost bin or be mixed in with the dirt in your garden. It is not advised to leave the cornmeal in the sandbox as it might attract rodents and other animals.

Clean up all your spills, wash out and dispose of the jugs and plastic sheet or tarp in the appropriate recycling bin (if you have one).

Analyze Your Data

If you videotaped the experiments, rerun your video .

See if your can time how long it takes for the jug to empty in the slow, medium and fast-flowing river using your video. If so, you can use these measurements to calculate the speed of each river by dividing the amount of water used (¾ gallons) by the number of seconds it took for the jug to empty.

Look at the details in your video and see if you can complete your observation tables similar to Table 1 in your notebook.

Compare your observations and measurements; can you see any trends? Here are some things to look for:

Did any of the rivers (slow-, medium-, or fast-flowing) show more/less erosion and/or deposits at specific distances from the source than the other types?

Did any of the rivers show evidence of becoming a braided river, a meandering river, or island formation?

Did any of the rivers show evidence or flooding or drought (water not reaching the end of the river)?

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Ask an Expert

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